Plastic containers or bottles are prevalent today in a wide variety of shapes and sizes for holding many different kinds of materials such as light duty liquids (e.g., dishwashing detergent), heavy duty liquids (e.g., laundry detergents), motor oil, vegetable oil, herbicides, etc. Generally, these containers are fabricated from layers or a plurality of layers of plastic, particularly polypropylene, polyethylene and polyesters, particularly poly(ethylene terephthalate), by means of blow molding or injection molding.
Generally such containers are provided with a label which designates the trade name of the product and may contain other information as well. In some instances, the label is merely attached to the container after molding by means of adhesive or the like. However, the label may also be attached to the container during the container molding process. This technology by which the label is associated with the container during the molding operation is generally referred to as an in-mold label process.
Many devices are known for performing in-mold labeling of a plastic container. For example, German Published Patent Application No. 1,807,766 to Rosler et al in 1969 shows making plastic containers labeled by in-mold techniques with a transparent plastic film reverse-printed with indicia viewable through the film. There is no adhesive layer between the label and the hollow plastic container, however, and the bottle label relies on the attraction between the polymeric label film and the plastic container body for adhesion. This means of adhesion, particularly in squeezable containers, has been a continuing problem, because such labels will not remain adhered to the container but instead they will split or separate because the finished container is normally bent, flexed and squeezed during use.
In U.S. Pat. No. 4,837,075 issued to Dudley in 1989, there is shown a coextruded plastic film label for in-mold labeling comprising a heat activatable ethylene copolymer adhesive layer and a surface printable layer comprising polystyrene with optional intermediate layers to provide interlayer adhesion and recycle of reground labels. The label has preferably a thickness in the range of from about 0.002 to 0.005 inches (0.05 to 0.127 mm). Because the layers have different refractive indices and to hide blemishes, the patent (Col.3, lines 46-60) teaches the need to add pigment or fillers to provide a suitable background for printing. This has the effect of making the label opaque and it will no longer be suitable for example to reverse print the back side of the label and thereafter view the printed indicia therethrough.
In published European Patent Application No. 0,281,701 issued to Court et al in 1988, there is disclosed an in-mold label formed from a thin sheet of multicellular thermoplastic film composed of a biaxially extruded opaque, non-transparent polyethylene/polystyrene copolymer for use with a blow molded plastic container. The in-mold label is said to resist curling, wrinkling and crazing, to have thermodynamic properties similar to those in the plastic of the container, and may be recycled along with the container. Such multicellular films do not, however, have the same specific gravity as the plastic from which they are made because they are filled with gas-containing voids. Furthermore, because the films are not transparent, translucent or even contact clear they must be printed on the outside and then overcoated with a surface coating to insure that the printed indicia does not smear or rub off (Page 4, lines 46-47). Printing is suggested on the inner surface of the backing (Page 5, lines 16-18), but only if the label is to be used as a peel-off label apparently because in such event the indicia cannot be read through the opaque label material. Such labels have the further disadvantage that they cannot be applied to containers and then reground in high volume after use to make recycled materials for blow molding containers because the container colors will not match the same color printed on the label.
The present state of the art is further evidenced by Modern Plastics, September, 1990, in an article relating to plastics used for labels, pages 83-85. It is stated therein that labelstock producers are still going after in-mold labeling with a variety of products. Among the materials under active investigation was mentioned paperlike polyolefin films. However, work has shown that such films are only commonly manufactured in thicknesses of up to about 0.0017 mil and they therefore are not thick enough to provide the necessary strength to withstand the heat of the molding process, even if they are reverse printed to resist staining from spillage and even though such materials assist in recycling. They are also too thin to pick and place in the mold.
It has also been disclosed in published World Patent No. WO 93/09225, May 27, 1993, that transparent, translucent, clear or contact clear polymeric films having judiciously selected characteristics of thickness, specific gravity and coefficient of expansion and contraction and provided with a heat activatable adhesive coating have improved and surprising characteristics of adhesion to in-mold blown plastic containers with resistance to damage from cracking, tearing, creasing, wrinkling or shrinking due to physical abuse and flexing of the plastic container material. Furthermore, if such sheets or rolls are reverse printed on the back before being overcoated with the adhesive, and labels made therefrom are then affixed to bottles during the blow molding process, abrasion scuffing and product spillage will not adversely affect the function of the label.
Unfortunately, such labels have some important drawbacks, one of which is a lack of adaptability to common printing techniques, such as gravure printing, flexographic and/or rotary screen printing, as well as combination of flexographic, rotary screen, and gravure printing techniques. Merely by way of example, experiments have shown that the required sheeting step is not possible on the gravure press or on the off-line sheeter because the smooth surface of the film is so charged with static through the process that the sheets cling together by static charge and jam the delivery mechanism or stack poorly rendering them useless without significant manual labor. This results in tremendous waste and added cost which is prohibitive for market entry. It has now been found that application of a coating of antistatic and/or slip agent either by deposition or coextrusion during substrate manufacture opposite to the adhesive face permits the sheets to be converted without such difficulty. Furthermore, numerous experiments have shown that it is difficult to "jog" the sheets square (i.e., to register them) for guillotine cutting into smaller sheets. It is so difficult to do so that the sheets must be registered manually (i.e., one at a time) which tedious procedure is not efficient enough for entry into the commodity sensitive label market. It has now been discovered that a judiciously applied antistatic and/or slip coating or coextruded layer makes it easy to jog the smaller sheets so they can be die cut to register for proper graphics. The next step of the process is the die cutting step where the shape of the label is die cut out of the smaller stacks of sheets. Alternatively, the printed sheets or rolls can be cut into individual labels by rotary die cutting where the antistatic and/or slip layer is critical to collection and jogging for packaging after rotary cutting. In any event, the stacks of printed labels in practice are impossible to pick and individually place in the mold with high speed machinery without a substantial number of misses or picking doubles. It has now been discovered that this drawback can be overcome by applying an antistatic and/or slip coating on the original feed stock opposite the adhesive layer. Although a number of antistatic agents are known, many experiments were required to discover suitable ones for the present purposes. It appears that the final coating must provide either a high slip surface in order to reduce the ability to form a static charge by providing a low coefficient of friction on the non printed surface, or a conductive agent to conduct away any generated charge, or, preferably, a combination of both. Of course, the antistatic slip layer must be one which is compatible with the printing method used, as opposed to, e.g., a heat- or melt-activatible material which will activate under mold conditions. It should also be understood that conventional direct printed labels for in-mold use do not have the problems associated with reverse printed labels because the former use inks and varnishes which provide some slip and they do not generate the static charges that reverse-printed smooth base films, especially polyester and polyolefin films do.
Accordingly, a principal object of the present invention is to provide for the use of reverse-printable polymeric sheets or rolls to make labels for in-mold use without the problems discussed above. It is a further object of the invention to provide a method for in-mold labeling of hollow plastic containers using printed labels made from such sheets. It is still another object of the invention to provide articles labeled with printed labels which have the unexpectedly superior properties described above.
These and other objects of the invention will become apparent from the present specification.